A tugboat used to help oil tankers travel safely through Alaska's Prince William Sound has run aground at the site of the Exxon Valdez disaster and is leaking diesel fuel, the U.S. Coast Guard said on Thursday.

The tug, called the Pathfinder and owned by Crowley Maritime Corp, hit Bligh Reef on Wednesday evening. The submerged reef, a notorious navigation hazard, is where the Exxon Valdez ran aground in 1989, leaking 11 million gallons (42 million liters) of crude oil in the country's worst oil-tanker spill.

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Fuel tanks holding up to 33,500 gallons (127,000 liters) of diesel were breached in the grounding, and aerial surveys show that a sheen about three miles long and 30 yards (meters) wide has formed, Coast Guard Chief Petty Officer Dana Ware said.

Tanker traffic out of Valdez, site of the marine terminal for the Trans Alaska Pipeline System, was temporarily interrupted but resumed Thursday morning, Ware said.

The 136-foot (41-meter) tug, part of the tanker-escort system set up after the Exxon Valdez disaster, had been scouting the tanker route for ice buildup, the Coast Guard said. It was not escorting a tanker at the time.